1. Field of the Invention
The invention relates to a process for separating off 1-amino-4-bromoanthraquinone-2-sulphonic acid, or the akali metal salts thereof, from reaction mixtures obtained from the bromination of 1-aminoanthraquinone-2-sulphonic acid in sulphuric acid.
2. Discussion of Prior Art
1-Amino-4-bromanthraquinone-2-sulphonic acid (termed bromamine-acid in the text which follows) is an important intermediate product for anthraquinone dyestuffs (compare Ullmans Encyklopadie der technischem Chemie (Ullman's Encyclopedia of Industrial Chemistry), 4th edition, volume 7, pages 639-640). It is prepared by sulphonating 1-aminoanthraquinone with sulphur trioxide or chlorosulphonic acid in sulphuric acid or in the presence of inert organic solvents (German patent specification Nos. 263,395 and 484,997, U.S. Pat. No. 3,428,659 and Japanese Published Specification No. 49,076,848) and brominating the resulting 1-amino-anthraquinone-2-sulphonic acid in an aqueous or aqueous-organic medium or in sulphuric acid (FIAT 1,313 II, page 214, U.S. Pat. Nos. 2,413,790 and 3,428,659 and Japanese Published Specification No. 49,076,848).
In the case of bromination in sulphuric acid in accordance with the Japanese Published Specification, the reaction mixture is worked up by dilution with a large excess of water. The bromamine-acid is precipitated in the form of its alkali metal salt by adding an alkali metal hydroxide solution and this salt is then isolated by filtration.
This working up has the disadvantage that filtrates with a high salt load are obtained, which represent a considerable load on the effluent. Processing of effluents having a high salt load is possible, for example by evaporation. However, this process not only involves a great deal of technical effort but large amounts of a residue of salt contaminated by organic substances are also formed. Thus, for example, when Example 1 of Japanese Pubished Specification No. 49,076,848 is repeated, about 7 kg of sodium sulphate or sodium bisulphate, per kg of bromamine-acid, are obtained as a result of neutralizing the sulphuric acid with sodium hydroxide solution.
A further disadvantage of working up by dilution with a large excess of water is that not only the bromamine-acid but also by-products of the sulphonation and the bromination, such as, for example, other sulphonic acids and 1-amino-2- and -4-bromo-anthraquinone and also, in particular, 1-amino-2,4-dibromoanthraquinone, are precipitated. These impurities are troublesome in most reactions of bromamine-acid to give dyestuffs and must therefore be removed previously.
According to the information in Japanese Published Specification No. 49,076,848, it is possible to remove the constituents which have not been sulphonated, such as 1-amino-2,4-dibromoanthraquinone, by shaking thoroughly with o-dichlorobenzene and separating off the organic phase. With this method, not only does the phase separation constitute a process step which is problematical from the technical point of view but it is also necessary, in order to keep the waste water pure, to free the aqueous phase from the proportions of organic solvents, which are always present, by distillation.
A further possibility for removing impurities is filtration to clarify the solution (compare, for example, Houben-Weyl, Methoden der organischen Chemie (Methods of organic Chemistry), 4th edition, volume I/1, page 185 et seq). For this purpose, the bromamine-acid precipitated from the reaction mixture is dissolved in hot water, active charcoal and/or a filter aid is added, the mixture is filtered and the bromamine-acid is again precipitated from the filtrate with the aid of alkali metal salts. Frequently, the required quality is obtained only by clarifying twice, for example in a weakly acid or weakly alkaline medium. The clarification process thus involves a considerable expenditure of time and on technical equipment and considerable costs. Moreover, large volumes of effluents which have to be processed are formed.
Broadly, this invention contemplates a process for separating off 1-amino-4-bromoanthraquinone-2-sulphonic acid from a reaction mixture formed during the preparation thereof by bromination of 1-aminoanthraquinone-2-sulphonic acid in sulphuric acid, which process comprises adjusting the sulphuric acid concentration of the reaction mixture under the prevailing conditions of temperature and pressure to effect precipitation of 1-amino-4-bromoanthraquinone sulphonic acid as the sulphate and thereafter separating the sulphate precipitate from the reaction mixture.